During a surgical operation it is usually easy to measure the extent of blood loss and to compensate it. So, at the end of the operation the blood volume of a person is normal. After every operation there is, depending on the state and medication of the patient and the type of the operation, at least some blood loss. Mostly this blood loss is so slight that no actions are needed. Postoperatively, in the recovery room, it is not easy to estimate a possible decrease in blood volume. At present, in most recovery rooms, a nurse measures the blood pressure and the heart rate of the patient for instance every 15 minutes. This takes quite a lot of time, and more personnel are needed. On the other hand, it is well known that blood pressure and heart rate are normal when the blood volume loss is not more than 10 percent of the normal blood volume. About 10 percent of the total blood volume of a person may be removed with almost no effect on either arterial pressure or cardiac output. In blood donation, about 500 ml of blood is removed with no harm to the donor. For a man weighing 70 kg this means 10 percent of his total blood volume. It is desirable to detect a blood loss condition while still in the early stage, because such an early detection would give hospital personnel more time to take corrective action. A problem is, however, that there is virtually nothing that would serve as an indicator of such early stage of blood loss.
Brody (1) published in 1956 a theoretical analysis postulating that the relatively large conductivity of the intracavitary blood mass augments the radial components of myocardial doublets. Published studies supporting the theory of Brody have been based on mathematical models (2-3), experimental animal investigations (4-6) and clinical studies (7-9). On the other hand, mathematical models (10), and clinical investigations (11-14) have shown an inverse relationship between ventricular volume and electrical voltage.
In animal experiments, withdrawal of blood has been used to modify cardiac volumes (4-6). In humans Valsalva maneuver (8-9), infusion of drugs (8, 13), sauna bathing (11), pressure cuffs around the limbs (7) and lower body negative pressure (14) have been used. Studies of the quantitative effects of acute hemorrhage on QRS and T wave amplitudes in humans was not found in recent literature.
Intensive experimental work to develop methods for noninvasive detecting a decrease in blood volume has been made from the beginning of the twentieth century.
Many physiological measurements have been used for detecting blood volume changes including impedance changes, an electrocardiogram, a photoplethysmogram and oxygen saturation, respiratory, skin temperatory and blood pressure measurements.
ECG changes exist always, if the decrease of blood volume is large. Unfortunately, the situation of the patient is already serious when ECG changes occur. Therefore, ECG changes occurring after smaller decrease of blood volume have not been looked for. Heart rate variability (HRV) that is dependent on the autonomic nervous system and not specific for blood volume is one of the most used ECG measurements. Studies using ECG amplitude measurements have been rare. No T wave or chest lead amplitude changes have been used. The probable reason for this is that reliance on chest lead amplitude measurements has been criticized because of great variations in serial measurements depending on non-standardized placement of chest electrodes (16). Also lower body negative pressure (LBNP), used in most studies to simulate decrease of central blood volume, can change ECG amplitudes by changing position of the heart and reducing the electrical conductivity of upper body by causing fluid shift from upper to lower body.
Another popular measurement for predicting blood volume changes has been blood pressure curve using photoplethysmogram. This measurement is partly dependent on left ventricular end-diastolic volume (preload), which reflects blood volume. It is, however, also dependent on the resistance of circulation (afterload). For instance the tone of autonomic nervous system, possible stenoses in arteries and position of the upper arm (if the probe is on a finger) acts on the resistance of circulation.
In most prior experiments LBNP has been used to simulate decreased central blood volume. The reason for this is that the ability to experimentally study the effects of blood volume decrease is currently limited to human studies in which blood loss is induced by voluntary blood donation. Members of US Army Institute of Surgical Research wrote in an article: “In the case of human blood donation, the removal of only small percentages of total blood volume is easily compensated for in most cases and does not provide predictions as to the point at which blood pressure decompensation will occur with greater volumes of blood loss” (Cooke W, Ryan K, Convertino A. Lower body negative pressure as a model to study progression to acute hemorrhagic shock in humans. J Appl Physiol 2004; 96: 1249-61). For the above reason the blood volume decrease in experiments using LBNP has been 15-20 percent or more in about 75 percent of test subjects.
Prior methods are relatively complicated using several different measurements and devices which raise the costs of the method.
A problem underlying the invention is related to the difficulty of detecting small decrease of blood volume. Currently there is no easy and noninvasive method to measure small changes of blood volume.